Python: matplotlib is producing no line in plot

Question

I'm running a simple bit of code to produce a plot similar to this. However the plot I am getting from my code is empty - I assume this is a fairly simple error in my code as I'm new to this. My code is:

import matplotlib.pyplot as plt
import numpy as np

omega_0 = 0.6911
w_0 = -0.77
a = []

for a in range(0,1,100):
omega_phi = (omega_0*(a**(-3.0*w_0)))/((omega_0*(a**(-3.0*w_0))) +    1 - omega_0)
w = w_0 + (((w_0*(1.0 - w_0**2.0))/(1.0 - 2.0*w_0 + 4.0*w_0**2))*((omega_phi/(1.0 - omega_phi)))) + ((((-1.0)*w_0*(1.0 - w_0**2.0))/(1.0 - 3.0*w_0 + 12.0*w_0**2))*((omega_phi/(1.0 - omega_phi))**2.0)) + ((((1.0)*w_0*(1.0 - w_0**2.0))/(1.0 - 4.0*w_0 + 24.0*w_0**2))*((omega_phi/(1.0 - omega_phi))**3.0))
plt.plot(a,w)

plt.xlabel('a')
plt.ylabel('w')
plt.grid(True)
plt.show()

Any help is greatly appreciated.

You are not updating a anywhere and w is getting overwritten each iteration. — turnip
– turnip, Commented Mar 15, 2018 at 15:28

ImportanceOfBeingErnest · Accepted Answer · 2018-03-15 16:25:08Z

1

range(0,1,100) gives you a single point. But you cannot draw a line through a single point. Possibly you want range(0,100,1), instead, which gives you 100 points between 0 and 99, or np.linspace(0,1,100), which gives you 100 points between 0 and 1.

Next you better use numpy to calculate your values.

Finally, consider simplifying your equations a bit, such that they become readable.

import matplotlib.pyplot as plt
import numpy as np

omega_0 = 0.6911
w_0 = -0.77
a = np.arange(0,100,1) # or np.linspace(0,1,100) depending on what you want.
om = omega_0*(a**(-3.0*w_0))
omega_phi = om/(om + 1 - omega_0)
p = w_0*(1.0 - w_0**2.0)
q = omega_phi/(1.0 - omega_phi)
w = w_0 +   p/(1.0 - 2.0*w_0 +  4.0*w_0**2)*q      - \
            p/(1.0 - 3.0*w_0 + 12.0*w_0**2)*q**2.0 + \
            p/(1.0 - 4.0*w_0 + 24.0*w_0**2)*q**3.0

plt.plot(a,w)

plt.xlabel('a')
plt.ylabel('w')
plt.grid(True)
plt.show()

edited Mar 15, 2018 at 16:25

answered Mar 15, 2018 at 15:36

ImportanceOfBeingErnest

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Comments

Laurent H. · Accepted Answer · 2018-03-15 16:40:16Z

0

I suggest you the following update of your original code. I assumed you wanted to display the curve with a going from 0 to 1 by 0.01 step.
I also added some comments for readability and for you to better understand the code:

import matplotlib.pyplot as plt
import numpy as np

# Define constants
omega_0 = 0.6911
w_0 = -0.77

# Set 'a' values from 0 to 1 by 0.01 step
a = np.arange(0, 1, 0.01)

# Set 'w' values associated with each 'a' value
omega_phi = (omega_0*(a**(-3.0*w_0)))/((omega_0*(a**(-3.0*w_0))) +    1 - omega_0)
w = w_0 + (((w_0*(1.0 - w_0**2.0))/(1.0 - 2.0*w_0 + 4.0*w_0**2))*((omega_phi/(1.0 - omega_phi)))) + ((((-1.0)*w_0*(1.0 - w_0**2.0))/(1.0 - 3.0*w_0 + 12.0*w_0**2))*((omega_phi/(1.0 - omega_phi))**2.0)) + ((((1.0)*w_0*(1.0 - w_0**2.0))/(1.0 - 4.0*w_0 + 24.0*w_0**2))*((omega_phi/(1.0 - omega_phi))**3.0))

# Plot the curve with all (a,w) points
plt.plot(a,w)
plt.xlabel('a')
plt.ylabel('w')
plt.grid(True)

# Show the curve
plt.show()

edited Mar 15, 2018 at 16:40

answered Mar 15, 2018 at 15:44

Laurent H.

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4 Comments

Paul H Over a year ago

if you're using numpy arrays, you don't need to iterate

Laurent H. Over a year ago

@PaulH You're right, sir ! Thanks a lot, I have updated my post with this simplification.

Brudalaxe Over a year ago

@LaurentH. thanks for the reply, this gives me a similar plot to the one I'm trying to get but it isn't quite the same - I think maybe I'm trying to solve the w equation in the wrong way

Laurent H. Over a year ago

@Brad You're welcome. You should accept the answer (mine or another) if it solves the question you asked in your original post.

turnip · Accepted Answer · 2018-03-15 15:43:12Z

0

There are several things going on. Firstly, you should not be calling plot in your loop. You call it once, where the arguments are your data points, in this case a and w. These should be lists.

Secondly, your range parameters were wrong; they should be `range(start, stop, step). In your code, you were going from 0 to 1 in with a stepsize of 1000, which would immediately go to 1. (I assume you wanted 1000 points from 0 to 1 instead? I've used simpler parameters.)

This should give you the desired plot.

import matplotlib.pyplot as plt

omega_0 = 0.6911
w_0 = -0.77
a = []
w = []

for i in range(0, 100, 1):
    omega_phi = (omega_0*(i**(-3.0*w_0)))/((omega_0*(i**(-3.0*w_0))) + 1 - omega_0)
    w.append(w_0 + (((w_0*(1.0 - w_0**2.0))/(1.0 - 2.0*w_0 + 4.0*w_0**2))*((omega_phi/(1.0 - omega_phi)))) + ((((-1.0)*w_0*(1.0 - w_0**2.0))/(1.0 - 3.0*w_0 + 12.0*w_0**2))*((omega_phi/(1.0 - omega_phi))**2.0)) + ((((1.0)*w_0*(1.0 - w_0**2.0))/(1.0 - 4.0*w_0 + 24.0*w_0**2))*((omega_phi/(1.0 - omega_phi))**3.0)))
    a.append(i)

print w, a
plt.plot(a, w)
plt.xlabel('a')
plt.ylabel('w')
plt.grid(True)
plt.show()

edited Mar 15, 2018 at 15:43

answered Mar 15, 2018 at 15:31

turnip

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1 Comment

Brudalaxe Over a year ago

thanks for the reply, this doesn't really give me the plot I'm looking for (maybe I've been doing the maths wrong) but if I import numpy and just change the code to for i in np.linspace(0,1,100) I get a more similar plot to the one I'm trying to get

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